The present invention relates to methods and apparatus for simultaneously reading multiple tracks of an optical disk, and more specifically to methods and apparatus for use with a swing-arm type optical pickup.
Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. For example, the compact disk (CD) format, developed and marketed for the distribution of musical recordings, has replaced vinyl records. Similarly, high-capacity, read-only data storage media, such as CD-ROM have become prevalent in the personal computer field for the distribution of software and databases. And the DVD format may soon replace videotape as the distribution medium of choice for video information.
Physically, the information bearing portion of an optical disk consists of a series of pits, or bumps, arranged to form a spiral track. Data is encoded in the length of individual pits and the length of the space between pits. An optical pickup assembly reads the data by reflecting a laser beam off of the optical disk. Because the disk is rotated, the laser beam alternately reflects from the pits and the spacing between the pits. This causes discernable changes in the reflected laser beam which are detected and decoded to recover data stored on the optical disk.
As used herein, data track refers to a portion of the spiral data track corresponding to a single rotation of an optical disk. A drive capable of reading multiple data tracks simultaneously reads multiple such portions of the spiral track at once. For disks having multiple concentric spiral tracks, data track refers to one revolution of one of the concentric spiral tracks. For optical disks having concentric circular tracks, data track refers to one such circular track.
U.S. Pat. No. 5,793,549 to Alon et al., describes as optical disk reader that reads multiple data tracks simultaneously, for example, using multiple laser beams. The multiple laser beams, which may be obtained by splitting a single beam using a diffraction grating or by providing multiple laser sources, are focused on and aligned with corresponding tracks of the optical disk. The reflected beams are then detected and decoded. Thus, a disk rotated at 6xc3x97 the standard speed in a disk drive reading ten tracks at a time provides a data rate equivalent to a 60xc3x97 single beam drive, but without the complications associated with high rotational speeds.
In addition to being aligned with the data tracks, the beams in a multi-beam optical pickup must be maintained at specified distances from each other to avoid crosstalk and to properly align the beams with the detectors. These distances are determined by the spacing of the tracks, i.e., the track pitch, the magnification of the optics, and the size and spacing of the detectors used to read the information. Typically, the minimum spacing is greater than the track pitch, requiring the multiple laser beams to be spaced circumferentially as well as radially with respect to the optical disk.
The necessary spacing between beams may be decreased either by increasing the magnification of the optics or by decreasing the size and spacing of the detectors as described in allowed U.S. patent application Ser. No. 09/042,185, xe2x80x9cMETHODS AND APPARATUS FOR PERFORMING CROSS-TALK CORRECTION IN A MULTI-TRACK OPTICAL DISK READER BASED ON MAGNIFICATION ERRORxe2x80x9d now U.S. Pat. No. 5959953. Increasing the magnification of the optics reduces the optical efficiency of the system, and reducing the size of the detectors reduces efficiency and increases manufacturing cost. The spacing of the beams in a multi-beam system represents a tradeoff between these factors. When the size, sensitivity, and cost of photo detectors improve, it may be possible to reduce the spacing between the beams.
An exemplary multi-beam optical disk reader is described in commonly-assigned U.S. patent application Ser. No. 08/911,815, entitled xe2x80x9cINTEGRATED MULTI-BEAM PICKUP ASSEMBLY,xe2x80x9d which is incorporated herein by reference. The optical disk reader described therein includes a plurality of reading beams arranged in a single row. Co-pending, commonly-assigned U.S. patent application Ser. No. 08/912,881, entitled xe2x80x9cMULTI-BEAM OPTICAL PICKUP ASSEMBLY AND METHODS USING A COMPACT TWO-DIMENSIONAL ARRANGEMENT OF BEAMS,xe2x80x9d which is incorporated herein by reference, describes an optical disk reader including a plurality of reading beams arranged in a two dimensional pattern. To maintain the needed distances between spots projected onto the surface of the disk as determined by the beam spacing, the pattern of laser beam spots must have a specific orientation with respect to the radial direction of the disk.
To read different portions of an optical disk a mechanism is provided for positioning the optical pickup adjacent to the portion to be read. Swing arm, rack-and-pinion, screw drive, and linear motor systems for positioning the optical pickup are known in the art, and described, for example, in Compact Disk Technology, Nakajima, H. and Ogawa, H., translated by Aschmann, C., Ohmsha, Ltd., Japan, 1992, and The Compact Disk Handbook, Pohlmann, K., 2nd. Ed., A-R Editions, 1992.
Selection of a positioning mechanism involves tradeoffs between access speed, design complexity, and manufacturing expense. For example, rack-and-pinion and screw drives are relatively slow at positioning the optical pickup. However, because they are also inexpensive and robust, they are often used in consumer level applications. By comparison, linear motors provide faster positioning, but are complex and more expensive than rack-and-pinion mechanisms. A swing-arm type positioning mechanism, such as that shown in U.S. Pat. No. 5,828,644, provides rapid positioning and is less complex than a linear motor systems.
While it would be desirable to use a pivoting arm such as described in the foregoing patent, several drawbacks arise from attempting to use such technology in a multi-beam optical disk reader. For example, in an optical disk reader that uses multiple laser beams, the orientation of laser beam spot pattern would change when the swing-arm pivots. Consequently, the laser beam spots may not align with the respective tracks at some radial positions of the optical pickup. This effect may increase the number of read errors or reduce the number of tracks that may be read simultaneously.
It would therefore be desirable to provide methods and apparatus for keeping multiple reading beams aligned with respective tracks of an optical disk when employing a swing-arm mechanism for positioning an optical pickup.
In view of the foregoing it is an object of the present invention to provide methods and apparatus that keep multiple reading beams aligned with respective tracks of an optical disk when employing a swing-arm mechanism for positioning an optical pickup.
These and other objects of the present invention are achieved by providing methods and apparatus for compensating for the rotation of the laser beam pattern projected onto an optical disk caused by motion of a swing-arm. In a first embodiment, when the swing-arm is pivoted, the laser beam spot pattern is pivoted, or rotated, in an opposite direction to offset rotation caused by motion of the swing-arm. In a second embodiment, the swing-arm and optical pickup are constructed so that the orientation of the laser beam spot pattern does not change when the swing-arm is pivoted. And in a third embodiment, the spacing between the laser beams spots is adjusted to compensate for the rotation of the line of laser beam pattern.